1. Field of the Invention
The present invention generally relates to control for straddle-type vehicles. More particularly, the present invention relates to a control technique for preventing a wheel slip from increasing excessively, for example, a wheel spin during acceleration or a wheel lock during braking.
2. Description of the Related Art
Conventionally, in straddle-type vehicles represented by motorcycles, which are lighter in weight than four-wheel vehicles, or the like, a ground load of a wheel (vertical load applied by the wheel), i.e., a pressing force applied by the wheel on a ground surface, is smaller, but its driving power is relatively greater. Therefore, a wheel spin, to be precise, a spin of a drive wheel, tends to occur in the straddle-type vehicle during acceleration. If a driver applies a brake strongly during braking, a wheel may be locked. In that case, the attitude of the motorcycle may become unstable.
As a possible solution to the above, a traction control (hereinafter referred to as TRC control) is known, in which, when a slip of a drive wheel increases during acceleration of a vehicle and a wheel spin is about to occur, a driving power of a driving source such as an engine is reduced to suppress the slip. Also, an anti-lock brake system control (hereinafter referred to as ABS control) is known, if a slip ratio of a wheel increases and the wheel is more likely to be locked during braking, a hydraulic brake pressure is forcibly reduced to suppress the wheel sip.
In addition, the center of gravity of a straddle-type vehicle, including a passenger straddling a seat, is relatively high. Therefore, a load may shift to a greater degree in a forward and rearward direction (this load shifting is referred to as pitching motion), during acceleration or braking. To address this, a technique of a suspension control is known, in which the damping force of a suspension is adjusted to suppress a rapid change in the attitude of the vehicle, due to the load shifting.
Specifically, Japanese Laid-Open Patent Application Publication No. 61-249889 discloses a technique in which the damping force of a damper is increased when a deceleration is high in a motorcycle, to suppress diving (nose diving) of the front portion of a vehicle body. Japanese Laid-Open Patent Application Publication No. 2008-137573 discloses a technique in which the damping force corresponding to extension of a front wheel and the damping force corresponding to diving of a rear wheel are increased, to suppress a squat motion of a vehicle body, during abrupt starting, etc.
However, the known TRC control or ABS control reduces a force to be applied in a rotational direction to the wheel in response to the driver's operation of an accelerator or a brake, i.e., a driving force or a braking force. Therefore, there may be a chance that a desired driving power performance is not attained, or a baking distance of the vehicle increases.
The above stated suspension control can stabilize the attitude of the vehicle effectively, but cannot enhance the driving power performance and cannot reduce the braking distance. Because of this, even if the suspension control is used in combination with the TRC control or the ABS control, the above stated problems associated with the TRC control and the ABS control cannot be resolved.